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Planetary Motion
Planetary motion is defined as the way a planet moves through a solar system. Planets move two
distinctly different ways. First they revolve around a Sun in an orbit that is elliptical. Secondly, they
rotate on an axis. The whole solar system is also moving as a unit, revolving around the center of our
galaxy, the Milky Way, which is moving though the universe.
Rotation
The rotation of the planet on its axis varies from planet to planet. The rotational period is called a
“day” and on the Earth it is 24 hours in length. Other planets rotate at different speeds but their “day”
is defined in terms of “Earth” hours. The axis angle a planet rotates on also varies from planet to
planet. The Earth rotates on an axis that has a 23.5 tilt from vertical. This tilt combined with the
elliptical orbit gives the Earth different seasons.
Revolution
 as a revolution, differs as well for every planet. The length of
The movement around the Sun, known
time it takes to complete one revolution around the Sun measures a year for the planet. The path that
the planet travels around the Sun is elliptical and is explained by Kepler’s Laws of planetary motion.
There are 3 distinct laws that Kepler discovered. The first law states that each planet orbits the Sun in
an elliptical orbit with the Sun at one of the foci of the ellipse. The second law, called the law of equal
areas, states that a line which connects a planet and the Sun will sweep out equal areas during equal
intervals of time no matter the position of the planet in its orbit. The third law states that the square of
the time a planet takes to orbit the Sun is directly proportional to the cube of the semi-major axis (the
distance from the center of the ellipse to the ellipse along the major axis or average distance) of the
P2
P2
planet’s orbit. Mathematically this law can be represented by the formula 3planet  earth
, where “a” is
3
a planet aearth
the semi-major axis and “p” is the orbital time of the planet.

Planetary Speed
Planetary speed can be measured for the planet’s rotation and revolution. The motion of a planet can be
calculated by knowing how far the planet moves in a set interval of time. Using the Earth as an
example, its rotational speed (distance it rotates divided by the time it takes to rotate) is equal to
1037.5646 miles per hour (0.46 km/s). The revolutionary speed is harder to calculate as a planetary
body acts like a projectile under the influence of gravity. Meaning it is similar to a ball thrown upward
against gravity, it gradually slows to a stop and then its speed increases as gravity pulls it downward. A
planet’s revolutionary speed slows down as it moves away from the Sun, but increases as gravity pulls
it towards the Sun. Its average speed can be calculated by dividing the circumference of the elliptical
orbit by the time it takes the Earth to complete one revolution (total distance divided by total time).
The Earth’s average revolutionary speed is 66 410 miles per hour (29.7 km/s).
Note: As planets travel an elliptical orbit the distance from the Sun or foci varies. Astronomer’s often
use AU or astronomical units, which are average distances for planets to aid in making calculations.
An AU for the Earth is defined as 149,597,870 or the average distance the Earth is from the Sun.
For explanatory diagrams see attached file:
I_Sci_020_Planetary_Motion_Diagrams.doc
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